1. To evaluate new players.
2. To get some real action in before the season actually starts. (so that the starters acually look like staters when the real season starts)
3. The owners get to keep all revenue from pre-season, including local TV contracts. (no profit sharing)

A lot of these guys can be awesome on the practice field, but be duds when they're in actual game conditions. The team is trying them out to make sure they're keeping the right guys that will back up the starters and fit in to the overall vision of the team.